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Special Notes
4.4
Evaluation of the Analog to Digital Converters(ADCs)
When using the F280049 on-chip ADCs there are some useful guideline to follow to realize the
performance numbers listed in the datasheet. This is especially true for the AC parameters such as: SNR,
THD, and SINAD. Furthermore, it can also be shown that there is a direct correlation between the SNR of
the ADC result and the spread of ADC codes seen for a DC input; as such these tips will improve the
range and standard deviation of a DC input as well. Finally, while topics addressed will be with respect to
the controlCARD, they are applicable to other implementations using the F280049 MCU as well.
On-board resistors and capacitors: By default
0-Ω shunt and all capacitors to the ground plane are not populated. While this circuit can be used to
supply the ADC inputs with a voltage, likely both the resistor(R) and capacitor(C) will need to be populated
based on the voltage source's characteristics. Referring to the
own RC network made up of the internal sample and hold capacitor, switch resistance, and parasitic
capacitance. By changing the inline resistance and parallel capacitor we can optimize the input circuit to
assist with settling time and/or filtering the input signal. Finally, it is recommended in general to use either
NPO(Negative-Positive 0 PPM/°C) or COG(Ceramic On Glass) as these have better stability over
temperature and across input frequencies than other types of capacitors.
Figure 5. Partial Schematic Showing Default R and C values
Voltage source and drive circuitry:While the on-chip ADCs are 12-bit architecture (4096 distinct output
codes when converting an analog signal to the digital domain); the translation will only be as precise as
the input provided to the ADC. The typical rule of thumb when defining the source resolution to realize the
full specification of an ADC is to have a 1-bit better source than the converter. In this case that would
mean that ideally the analog input should be accurate to 13-bits.
Typically voltage supplies or regulators are not designed to be precise, but rather accommodate a wide
range of current loads within a certain tolerance and for this reason are not ideal to show the performance
of a higher bit ADC, like the one on the F280049. This does also not take into account that many times the
supply in question is providing the main voltage to power the MCU itself; which also introduces noise and
other artifacts into the signal.
In addition to the quality of the input signal there is also the aspect of the load presented to the ADC when
it samples the input. Ideally an input to an ADC would have zero impedance so as not to impact the
internal R/C network when the sampling event takes place. In many applications, however, the voltages
that are sampled by the ADC are derived from a series of resistor networks, often large in value to
decrease the active current consumption of the system. A solution to isolate the source impedance from
the ADC sampling network is to place an operational amplifier in the signal path. Not only does this isolate
the impedance of the signal from the ADC, it also shields the source itself from any effects the sampling
network may have on the system.
8
Piccolo F280049 controlCARD Information Guide
(Figure
Copyright © 2017–2019, Texas Instruments Incorporated
5) all inline resistors to the ADC pins are a simple
ADC Input
Model, the ADC input has its
SPRUIC4C – July 2017 – Revised January 2019
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